Yahoo captured the top spot from rival Google for the first time in the University of Michigan's American Customer Satisfaction Index report on electronic-business Web sites. Yahoo's customer satisfaction score of 79 on the ACSI's 100-point scale rose almost 4% this year, while Google slipped 3.7% to 78, its second yearly decline in a row.   Ask.com scored 75, this year's biggest increase and Time Warner Inc.'s AOL.com the biggest decline, down more than 9% to 67. Microsoft Corp.'s MSN.com, with a 75 score, is up only one point this year.  Claes Fornell, head of the ACSI, said in an interview Monday that Google may be falling behind because of a lack of innovation in the basic product it offers. The University of Michigan professor said the Google interface has changed very little even as the user base has exploded. Yahoo, meanwhile, has added several features. The survey comes as Yahoo and Google battle for online-advertising revenue. Yahoo last month vowed to refocus its business under new Chief Executive Jerry Yang as it conceded strategic mistakes have hobbled its performance.